Method of coating by vapor deposition



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Jan. 5, 1954 W. SCHULER E AL METHOD OF COATING BY VAPOR DEPOSITION Filed Nov. 5, 1951 IN VEN TORS Wes/2y 14/. Mansir William, O. DI'P/efro ATTORNEY;

Patented Jan. 5, 1954 UNITED STATES PATENT OFFICE METHOD OF COATING BY VAPOR DEPOSITION Application November 5, 1951, Serial No. 254,936

1 Claim. 1

This invention relates to coating and more particularly to the continuous, high-vacuum, vapor deposition coating of metals, such as aluminum, on flexible substrates.

A principal object of the present invention is to provide improved coating processes and apparatus for continuously applying a coating of aluminum to a flexible substrate by vapor deposition techniques.

Another object of the invention is to provide improvements in such processes which give high coating rates with continuous operation of the aluminum vapor source over a long period of time.

Still another object of the invention is to prevent localized erosion of an elongated carbon rod which is utilized as the aluminum vapor source.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others, and the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claim.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

Fig. 1 is a diagrammatic, schematic, sectional view of one preferred form of apparatus embodying the present invention; and

Fig. 2 is a diagrammatic, schematic, sectional view of Fig. 1 taken along the line 2-2, portions of the apparatus of Fig. 1 being eliminated from Fig. 2 for simplicity of illustration.

In general the present invention relates to vapor deposition coating and more particularly to coating processes and apparatus of the type shown in the copending Clough et a1. application, Serial No. 171,432. In common with the above Clough application, the present invention permits high coating speeds over long periods of operation, the source of aluminum vapors having a large effective evaporating surface and being capable of operation at high temperatures. These features of the present invention are particularly useful during continuous coating of heat-sensitive substrates. The present invention constitutes an improvement in the invention described in the above identified Clough application in that it prevents gradual erosion of a dense carbon rod which is utilized as the source of aluminum vapors. This erosion is particularly apparent where high rates of aluminum evaporation per unit of rod length are required. This is due to the fact that with high evaporation rates, large quantities of aluminum must be fed to the rod and, the longer the rod, the more aluminum that must be fed thereto.

When utilizing a long carbon rod it is preferred that the aluminum be fed to the rod at some point along its length. For ease of control it is preferred that this feeding be adjacent one or both ends of the rod, the aluminum being preferably melted as it contacts the rod and flowing along the rod to fill a groove which is preferably provided in the top of the rod. In the above Clough application, attack of the major surface area of the rod has been quite effectively prevented by providing on the rod a surface stratum of a carbide of one of the group IV'a and group Va metals (i. e., titanium, zirconium, hafnium, vanadium, columbium, and tantalum). While such carbide surfaces are relatively inert to molten aluminum, even at high temperatures, and are readily wet by molten aluminum to assure complete coverage of at least the upper portion of the rod surface, it has been found that gradual attack of this carbide coating takes place at the point where the aluminum is fed to the rod. This is believed to be due to the fact that there is some slight solubility of the carbide surface stratum in the molten aluminum. While this solubility is not much, fresh aluminum is continuously being fed to a single point on the rod and is continuously flowing past this point. Thus, even a very small percentage solubility of the protective carbide in the fresh aluminum can cause considerable erosion over a long period of operation. In the operation of the rod it has been found that the carbide surface stratum appears to be transferred along the rod in the direction of flow of the aluminum, it being redeposited further along the rod as the aluminum is evaporated therefrom.

In the present invention, erosion of the rod at the point of feed of the aluminum is prevented by initially providing on the carbon rod a surface stratum which is wettable by the molten aluminum and only slowly eroded by the molten aluminum. There is also fed to the rod, adjacent the point at which the aluminum is fed thereto, an appreciable quantity of a second metal which is soluble in the molten aluminum. This second metal is of the class consisting of titanium, zirconium and tantalum. For simplicity of description, this second metal will be referred to hereafter as titanium, without intent to limit the scope of the invention. This titanium, when being fed with the aluminum, can be in the form of an aluminum-titanium alloy. Where an alloy is employed, the titanium is preferably a very minor portion of the alloy, being in the range, by weight, of about 1% to this range being preferably between about 2.5% and 5%. Alternatively, the titanium may be fed, as a solid, separately from the aluminum. In this case the aluminum may be fed as a wire to a predetermined point on the rod and a titanium wire is preferably fed so that it strikes the rod at a point closely-adjacent to the point at which the aluminum wire strikes the rod. In this case the titanium wire is fed at a rate sufficient to maintain between about 1% to 5% by weight of the titanium dissolved in the aluminum adjacent the point offeed thereof.

ferring to 1 and 2 there is shown one diagrammatic representation of one embodiment of the invention. In these figures, If} represents a vacuum-tight housing defining therewithin a vacuum coating chamber [2 which is arranged to be evacuated to'a low free air pressureon the order of less than a micron Hg abs. by means of a-vacuum pumping system schematically indicated at i l. Within this chamber the substrate iii to be coated is 'guided'from a supply ii thereof past a plurality of cooled guiding rolls 1% to a take-up spool I 9. During the process of the passage from supply I! to take-up spool IS, the substrate It travels in a series of convolutions near a source Zilof aluminum vapors. This general arrangement of substratefeed and'guiding rolls is similar to that illustrated in the above mentioned Clough et a1. application, Serial'No. 171,432. source 29 preferably comprises'a-dense carbon rod 22 having a groove 24 in the upper'surface thereof. In Fig.2 the-guiding rolls i3 havebeen eliminated so as to simplify the illustration of the source and the associated wirefeedingmeohanisms. This rod 22 is arranged to be fed by aluminum 26, this aluminum being fed directly into the groove 24 in the upper surfac thereof and preferably maintaining this groove full of molten aluminum. As illustrated in Fig. 2, aluinum is fed, in this embodiment of the'invention, to both ends of the rod, two aluminum wires being illustrated for this purpose. A second pair of wires 2? is also fed to the 0nd wires being titanium and being fed immediately adjacent the points of feed of the aluminum wires 25.

The carbon rod source 2%, which is generally of the type described in connection with the above mentioned Clough et a1. application, includes a pair of holders ZB-Which support the rod at the two ends and act as electrical connections for permitting high current to flow through the rod and aluminum carried thereby so as to heat the rod and the aluminum by resistance heating. These two rod holders 2B are schematically indicated as being cooled by water-cooling channels 3B, the water being fed to these channels by appropriate pipes shown at 32. For convenience these pipes 32 may also serve as the electrical current leads to the rod holders. The rod 22 is preferably notched at 34 adjacent each end so as to provide an area of high heating adjacent the water-cooled connections 28. The purpose of this high temperature area at. tilt @lld5 i the As shown more clearly in Fig. 2, the

groove '24, these ,eec-,

.up all of the aluminum in the groove 24.

. ..easuring rod is to prevent travel of the aluminum from the groove 24 to the rod holders 28 where it might freeze and build up a low resistance path. This aluminum. freezing action, due to the low resistance of the massive aluminum, could conceivably travel along the rod and thus freeze This isparticularly apparent when the ends of the red are maintained below the melting point of aluminum, as may be the case when the rod holdera-2'8 are made of copper and are connected .directly to the ends of the rod. Electrical leads 38 are preferably connected to the cooling water pipes 32, these l ads extending from a power source--48, shown as a low voltage transformer.

The aluminum 26 is preferably supplied from a pair of wire coils 42 thereof, while the second pair of titanium wires 2? are fed from coils 44 thereof. Two pairs of wire-feeding mechanisms 18 are provided for feeding the two pairs of wires adjacent the ends of the rod source These wire-feeding mechanisms include wire-guiding tubes 4? and, are arranged so that the aluminum wire 26 is fed at a much faster rate, by Weight, than the titanium wire 2?. As mentioned pre viously, this titanium wire fed at a rate so that the weight thereof being fed, a any instant of time, is only about 1% to 5% of the weight of aluminum being fed. In a preferred embodiment of the invention the wire-feeding mechanisms n; are preferablycontrolled by a control means 43', such as a rheostat. This control means 48 is in turn preferably controlled by a current device 56 (such as an emzneter) which is arranged to measure the amount of current flowing through the loads 36. The current measuring device 59 thus measures the amount of current flowing through the rod. 22 and the aluminum carried thereby.

In one preferred form of the invention the carbon rod is formed of Becker Bros. B carbon. The rod may be about /2 inch in diameter, about 6 inches long (five inches effective evaporating length) and may have a groove in the top thereof which is inch wide by about inch deep. Such a rod, with no aluminum, will be heated to about 1300" C. with a current of about 300 amperes at a voltage of about 11 volts. When the groove in the rod is filled with aluminum it will carry 600 to 700 amperes with a voltage drop of only 4 volts at an evaporating temperature of about 1300 C. It might be well to point out that a rod of only six' inches in length does not require feed of aluminum at both ends.

In the operation of the device shown in Figs. 1 and 2, a roll ll of the substrate It is positioned within the vacuum chamber [2. The substrate is guided around the various rolls l8 and is connected to the take-up spool I9. Two spools of aluminum wire 42 are positioned so that the ends thereof may be fed through their respective wire-feeding mechanisms 45 and through their wire-guiding tubes 6?. The two spools'of titanium wire 44 are similarly fed through the wirefeeding mechanism 48 and their guiding tubes 47. Cooling water or other refrigerant (at about 40 F. or lower) is then circulated through the rolls I8 by suitable piping (not shown) so as to chill the rolls l8. Cooling water is also circulated through the pipes 32 and the channels 30 in the rod holders 28. In those cases where the rod 22 is an uncoated carbon rod, pieces of solid aluminum and solid titanium may be inserted in the groove 24, theamount of aluminum being used in considerable excess over the amount of titanium (e. g., 96% aluminum and 4% titanium, by weight). Vacuum chamber i2 is then evacuated to a low free air pressure (on the order of one micron I-Ig abs.) by vacuum pumping means I4. When the requisite low pressure is achieved, rod 22 may be heated to a temperature considerably in excess of the melting point of aluminum (e. g., about 1100 C.) so that the molten aluminum dissolves the titanium in the initial charge of aluminum. During the initial evaporation of the aluminum placed on the rod, it is desirable to protect the substrate from radiation emanating from the source. This can be conveniently achieved by providing a removable shield (not shown) which is positioned over the rod source until the coating of the substrate is to begin. When some of this initial charge of aluminum has been evaporated, the shield may be removed and the movement of the substrate past the rod source 20 may be commenced, preferably by driving the take-up spool l9 and the various guiding rolls 18. The automatic control 48 may then be energized and, since a good percentage of the initial charge of aluminum in groove 24 will have been exhausted, the current measuring device 59 will indicate a relatively low current flow in, and a corresponding high resistance for, the rod 22 and the aluminum carried thereby. The current measuring device 58 will thus indicate a need for aluminum, and the control mechanism is will energize the wire feeding mechanism 46 to commence the feed of aluminum and titanium wires to both ends of the groove 2%. When sufficient aluminum has been fed to fill the groove 24, the current will increase and the feed of aluminum wire will be decreased or stopped. As the titanium wire is fed along with the aluminum, it is dissolved by the aluminum and is maintained in solution except to the extent that it reacts with the carbon in the rod to form titanium carbide at the interface beween the aluminum and the carbon. This reaction will take place wherever the molten aluminum, containing the titanium, strikes the bare carbon rod. Since titanium carbide is readily wet by the molten aluminum, the formation of the titanium carbide permits the aluminum to climb up the walls of the groove 2 and to spread over the top and side surfaces of the rod 22, the titanium carbide surface forming on the rod as the aluminum travels over the surface of the rod.

As mentioned previously, the feed of titanium wire is at a much lower rate, by weight, than is the feed of the aluminum wire. After the titanium carbide surface has been initially formed, the need for titanium decreases somewhat since this titanium carbide surface is only very slowly attacked by aluminum, particularly when the' aluminum contains some titanium in solution. However, the feed of titanium along with aluminum is desired, since at the point of feed of the aluminum the titanium content in the aluminum would tend to become extremely small, or essentially nonexistent, in the event that titanium were not fed adjacent this point. This is due to the fact that fresh aluminum flows from the point of feed thereof along the rod towards the center of the rod. When this rod has an appreciable length, on the order of from 12 to 24 inches, very large quantities of aluminum can be evaporated therefrom and, as a consequence, very large quantities of aluminum must be fed to the rod. This aluminum flowing from its feed point along the rod slowly dissolves the titanium carbide down the rod towards the middle therof where the titanium carbide is precipitated due to the evaporation of the aluminum. However. when titanium wire is fed along with the aluminum at the point of feed thereof, the aluminum, event at its point of feed, contains an appreciable quantity of titanium so that any titanium carbide which is dissolved by the aluminum is immediately replaced by new titanium carbide. Also it appears that the presence of titanium in the aluminum prevents attack of the titanium carbide by the aluminum. It is apparent that the aluminum is rapidly evaporated from the rod, while the titanium is only slowly evaporated, due to its much lower vapor pressure at the temperature involved (1300 to 500 0.). As a consequence, the concentration of titanium in the aluminum on the rod will tend to increase. Experimental evidence indicates that the excess titanium comes out of solution further down the stream of aluminum flow as the concentration of titanium in the aluminum becomes greater due to the evaporation of the aluminum. The thus deposited titanium also serves as a relatively inert surface to aluminum which contains a saturated solution of titanium. As a consequence the titanium serves the dual purpose of maintaining the titanium carbide rod surface intact and also of providing an overcoating of titanium which is inert to the aluminum as long as the aluminum is saturated with titanium.

While one preferred embodiment of the invention has been described above, numerous modifications thereof may be made without departing from the scope of the invention. For example, the carbon rod may be given a protective coat prior to installation in the coating apparatus, such as by coating with one of the metals from the class consisting of titanium, zirconium, hafnium, vanadium, columbium, tantalum, molybdenumand tungsten. This coating may be achieved by reduction of the corresponding oxides of these metals on the surface of the carbon rod so that the carbide of these metals is formed. Equally, the second metal may be applied to the carbon rod by other techniques The second metal carbide may be formed along with the reduction of the metallic compound or after the reduction of the compound by suitable heat treating under a high vacuum. Examples of such treatment are given in the copending application of Clough et al., Serial No. 171,432, and the copending application of Clough et al., Serial No. 231,916. 7

While titanium has been described above as being the preferred metal for feeding with the aluminum, zirconium and tantalum may be equally utilized. Also these metals may be fed as an alloy of aluminum, this alternative embodiment of the invention having the advantage that the aluminum wire 25 may carry the proper amount of the second metal (e. g., titanium) such as by being formed of an aluminum alloy containing about 1% to 5%, by weight. of the second metal. Equally two or more of these metals may be fed with the aluminum. For example, we have used an aluminum Wire containing about 3% titanium and about 2% zirconium alloyed therewith.

While the preferred embodiment shows the feed of aluminum to both ends of the rod 22, it is apparent that more or fewer feeding means may be employed. When the rod is relatively short, such as 6 inches, aluminum may be fed only to one end. When the rod is on the order of 12 to 24 inches,'feeding at both ends is preferred so as to assure complete coverage of the rod with aluminum at all times. With the longer rods aluminum may be fed intermediate of the ends, as well as the ends, if desired. In all cases, however, it is preferred that the second metal (i.-e,. titanium, Zirconium or tantalum) be fed along with the aluminum, either as an alloy with aluminum or as another wire, so as to prevent local erosion of the rod at the points of aluminum feed.

While a preferred embodiment utilizing wire feed has been employed for illustrating the present invention, it should be apparent that aluminum may be fed to the rod as a powder or as a liquid. In these modifications of the invention the second metal is fed to the rod adjacent the feed of the powdered or liquid aluminum. The titanium, or other second metal, can be fed as a powder, as a coating on the aluminum wire, or as a filament intertwined with the aluminum wire. A preferred embodiment of the invention, from the standpoint of simplicity of feed, comprises an aluminm. titanium alloy wire. Such a wire is conveniently prepared by melting aluminum, adding about 4% by weight of titanium thereto, casting a billet of the resultant titanium-aluminum alloy, and drawing the billet into a wire of the desired diameter. We have found that melting the aluminum in a vacuum, before adding the titanium, has been particularly helpful in outgassing the aluminum and thus seems to prevent spattering during subsequent evaporation.

In the preceding discussion of the invention the weight percentage of the second metal (i. e., titanium, zirconium, or tantalum) fed with the aluminum has been specified as being in the range of about 1% to 5%. The upper limit of this range can be exceeded but there is not any advantage in feeding a greater percentage of the second metal since it is, even at the relatively high temperatures involved (1300-1500 C.), soluble to the extent of only a few percent in the molten aluminum. The excess undissolved second metal is thus serving no useful function. In this connection it would be well to point out that one advantage of the use of an aluminumtitanium alloy (for example) is the fact that the titanium is already dissolved in the aluminum and there is no time lag necessary for the dissolution of the titanium.

Since certain changes may be made in the above process and apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

The process of coating a substrate with aluminum which comprises providing a vacuum chamber in which aluminum may be evaporated under a vacuum, positioning said substrate within said vacuum chamber, providing a support comprising elemental carbon, providing a supply of aluminum and a second metal on said support, said second metal being taken from the class consisting of titanium, zirconium and tantalum, heating said aluminum and said second metal to a temperature above the melting point of said aluminum, said second metal being provided in an amount by weight equal to about 4% of the aluminum on said support, thereafter substantially continuously feeding aluminum to said support as said aluminum evaporates feeding said second metal to said support separately and simultaneously with the aluminum feed, said second metal being fed at a rate such that its weight is between about 1% and 5% of the weight of aluminum being fed at any period of time, heating said support to a sufficiently high temperature to vaporize aluminum on said support, advancing said substrate past said support, and condensing on said substrate aluminum vapors emanating from said support.

FREDERIC VI. SCHULER. WESLEY W. NIANSIR. WILLIAM; O. DI PIETR References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,153,786 Alexander et al Apr. 11, 1939 2,282,098 Taylor May 5, 19%2 2,363,781 Ferguson Nov. 28, 1944 2,382,432 McManus et al Aug. 14, 1945 2,450,855 Colbert et a1. Oct. 5, 1948 2,557,530 Bancroft June 19, 1951 

